An implantable medical device may connect to one or more implantable conductors that are outside of the device. For example, an implantable cardiac rhythm management device (e.g., a pacemaker, a defibrillator, or a cardioverter) may connect to one or more leads implanted in or near the heart of a patient to monitor cardiac function and provide therapy for a patient who suffers from cardiac arrhythmia. For example, the implantable device may process signals received via implanted cardiac leads to track the type and timing of native cardiac signals. In the event cardiac events are not occurring at appropriate times or undesired cardiac events are detected, the implantable device may apply stimulation signals to the heart via the implanted cardiac leads in an attempt to restore normal cardiac rhythm.
Studies have found, however, that magnetic resonance imaging (“MRI”) scans of a patient may cause an implantable medical device implanted in the patient to malfunction. For example, in some cases time-varying magnetic fields generated during an MRI scan may induce currents in an implanted lead that may, in turn, stimulate (i.e., cause capture of) cardiac tissue. In addition, a relatively large current and/or voltage may be generated at an external interface of an implantable medical device as a result of MRI scanning. In such cases, signals may enter internal circuitry of the implantable medical device and cause the device to malfunction. In some cases, MRI-induced signals may cause heating problems in the implanted lead.
MRI-induced stimulating currents may arise in different ways. In some cases pulsed magnetic gradients used during MRI scanning may induce voltage in an implanted cardiac lead connected to an implanted device. If such voltage appears across sufficiently low impedance, current will flow which may stimulate the heart. In some cases, pulses of amplitude modulated radiofrequency (“RF”) energy from MRI scanning (e.g., with a carrier at 64 MHz or 128 MHz) may be rectified within an implantable device and exit the implantable device as a lower frequency demodulated signal on the implanted lead. This lower frequency signal may then travel to the patient's heart via the implanted lead and potentially cause unintended pacing. In some cases, this unintended pacing may cause cardiac fibrillation.
In view of the above, a physician may elect to not prescribe MRI scanning for a patient who has an implanted medical device. Consequently, such a patient may receive suboptimal treatment. Accordingly, a need exists for MRI-compatible implantable medical devices that are immune to the influence of MRI magnetic fields. Ideally, a patient who has an implanted MRI-compatible medical device should have no extra restrictions going under an MRI scan as compared to a patient who does not have such an implanted device.